Our invention relates to selective heating apparatus and, more particularly, to heating apparatus for soldering spaced electrical elements concurrently.
Soldering has long been used for joining electrical elements in electronic apparatus. More recently, batch soldering techniques have been developed to simultaneously join a large number of elements. In one arrangement known as reflow soldering, the surface of each element is precoated with solder. Subsequently, the precoated elements are brought together and heat is applied to cause the solder to reflow. The elements are thus joined after the solder cools.
Further developments in electronic circuit techniques have required concurrent soldering of a large number of spaced electrical elements, such as connector terminations, flat, flexible cable, and flexible circuits. U.S. Pat. No. 3,991,297, issued Nov. 9, 1976, to H. H. Ammann, and U.S. Pat. No. 4,081,658, issued Mar. 28, 1978, to H. H. Ammann, both assigned to the present assignee, disclose apparatus for reflow soldering a large number of spaced electrical elements. In accordance with the aforementioned patents, a heat generating member dimensioned to concurrently contact a plurality of spaced elements is connected between spaced, parallel, relatively massive and high conductivity bars. A section of the heat generating member between the bars is placed in contact with the surface of electrical elements that have been precoated with solder. Electrical current is directed uniformly through the heat generating member to provide a uniform temperature to all the spaced elements. The uniform temperature profile of the heat generating member is not, however, adapted to soldering spaced elements having significantly different thermal loads. After reflow occurs, the electric current is discontinued. The solder operation is complete when the solder cools.
The relatively massive bars help dissipate heat from the heat generating member during cooling. The rate of heat dissipation depends on the total amount of heat to be removed and the thermal efficiency of the bar structure. The bars must be large and heavy in order to achieve an acceptable rate of heat dissipation. The heavy bars and associated current supply cables, however, are inconvenient for accurate positioning of the heat generating member and for rapid cycling in production line operations. It is therefore an object of the invention to provide precise, repeatable positioning of the heat generating member. Another object is to provide controllable temperature profile soldering of a plurality of spaced elements at a high production rate. A further object is to provide simplified and replicable manufacturing of a soldering tool having a predetermined temperature profile.